The present invention relates to the art of microelectronics, and more particularly relates to leads and lead bonding methods suitable for forming electrical connections to microelectronic devices.
Microelectronic devices such as semiconductor chips and microelectronic circuit panels can be connected to one another by a variety of methods. In wire bonding, a component such as a semiconductor chip having contact on a front surface typically is disposed on the top or contact bearing surface of a circuit panel. The front surface of the chip having contacts thereon faces upwardly, away from the circuit panel. Fine wires are connected between the contacts on the chip and the contacts on the circuit panel by processes such thermosonic ball or wedge bonding.
In flip-chip bonding, the chip or other device is provided with its contact-bearing surface facing the contact-bearing surface of the circuit panel, so that each contact on the chip is aligned with a corresponding contact on the circuit panel. Individual solder masses are provided at each pair of aligned contacts and melted or xe2x80x9creflowedxe2x80x9d so that the solder wets the contacts on the chip and on the circuit, whereupon the assembly is cooled to solidify the solder and form permanent connections. In tape automated bonding, a flexible film or xe2x80x9ctapexe2x80x9d is provided with contact pads and with leads extending from the contact pads. The leads are arranged in a pattern corresponding to the contacts on the chip. The tape is placed over the chip and the individual leads are bonded to individual contacts on the chip. The resulting assembly can be bonded to a circuit panel by a process similar to that employed in flip-chip bonding. Thus, each contact pad of the tape is aligned with the corresponding contact on the circuit panel and a solder mass is provided between each pair of aligned contacts and reflowed to form a permanent connection.
As described in certain embodiments in certain U.S. Pat. No. 5,518,964, the disclosure of which is incorporated by reference herein, a connection to a microelectronic element such a semiconductor chip can be made by providing a connection component with a support structure such as dielectric layer and with a plurality of leads extending along a bottom surface of the support structure. Each lead has a terminal end permanently attached to the support structure and a tip end releasably attached to the support structure. The tip ends of the leads are arranged in a pattern corresponding to the pattern of contact on the microelectronic device. Typically, each lead has a mass of a bonding material such as a eutectic bonding alloy or solder at its tip end. The connection component is engaged with the microelectronic device so that the tip ends of the leads engage the contacts on the device. While the tip ends of the leads are engaged with the contacts on the device, the bonding material is activated, as by heating, to form a liquid phase at the interfaces between the tip ends of the leads and the contacts on the device. This liquid phase solidifies to form a permanent bond between the tip end of each lead and the associated contact. For example, where the bonding material is a eutectic bonding alloy, the alloy liquefies at a relatively low temperature when the assembly is heated during the bonding process. However, diffusion between the liquid phase and the adjacent solid phases of the lead and contact changes the composition of the eutectic alloy and raises its melting temperature to above the prevailing temperature, whereupon the liquid phase solidifies. Where the bonding material is a solder, the liquid phase forms when the assembly is heated to above the melting temperature of the solder and solidifies when the assembly is cooled to below the melting temperature of the solder.
In preferred embodiments according to the ""964 patent, after the tip ends of the leads are bonded to the contacts, the support structure and the device are moved away from one another so as to bend the tip ends of the leads away from the support structure of the connection component and thereby deform the leads to a vertically-extensive disposition. In this disposition, the leads are flexible. A dielectric material such as a compliant encapsulant may be provided between the support structure and the device, as by injecting a curable liquid material during or after the moving step. In variants of the processes taught by the ""964 patent, the bonding material can be carried on the contacts of the chip or other device, rather than on the tip ends of the leads. Preferred processes according to the ""964 patent provide extraordinarily useful methods for making connections. Merely by way of example, these processes can be used to connect all of the chips on a wafer to terminals or other conductive features on a support structure in a few steps. After these connections are made, the wafer and the support structure can be severed to provide individual units, each including one or more chips and the associated portions of the support structure.
Despite these and other developments in the art, still further improvement in processes using bonding materials to connect leads and contacts would be desirable. Improvements in components used in such processes also would be desirable.
One aspect of the present invention provides components for use in microelectronics. A component according to this aspect of the invention has a support structure as, for example, a dielectric element or an active electronic device such as a semiconductor chip or semiconductor wafer. The component also includes a plurality of leads. Each lead has a lead structure including a main region and a tip region which can be moved with respect to the support structure. The lead structure has a wettable surface in the tip region and a non-wettable surface bounding the wettable surface. The wettable surface is wettable by a liquid bonding material, whereas the non-wettable surface is not. Therefore, when the tip region of the lead is exposed to a liquid bonding material, the liquid bonding material wets the tip region but it does not spread from the tip region onto the main region. The lead may have a mass of solid bonding material on the tip region. The bonding material is adapted to form a liquid which wets the wettable surface but which does not wet the non-wettable surface.
For example, the lead structure may include a metal such as copper and the non-wettable surface may be formed from a compound of the metal such as a copper oxide. In one particularly preferred arrangement, the wettable surface is defined by an oxidation resistant metal such as a metal selected from the group consisting of gold, platinum and alloys thereof. The bonding material may a solder or may include a eutectic-forming metal adapted to form a low-melting eutectic with surrounding metals. Other bonding materials may be employed. In use, the leads can be bonded to contacts on a microelectronic device by engaging the tip ends of the leads with the contacts and heating the assembly. The bonding material forms a liquid phase which wets the tip end of the lead and which also wets the contact. This liquid phase however does not spread along the length of the lead. Instead, the liquid phase is confined at the tip of the lead by the non-wettable surface. Confinement of the liquid bonding material to the tip end of the lead produces several desirable results. First, because the liquid remains at the tip end of the leads, it is available to form the desired bond with the contact on the device. Also, the liquid bonding material cannot embrittle regions of the lead outside of the tip region. This advantage is significant because regions of the lead outside of the bond typically are exposed to greater flexural fatigue stresses during service.
Desirably, the main region of each lead includes an anchor end remote from the tip end, the anchor end of each lead being attached to the support structure. The connection between the anchor and the support structure may be the sole connection between each lead and the support structure. Alternatively, the tips of the leads or regions of the leads adjacent the tip ends may be releasably attached to the support structure.
A further aspect of the invention provides methods of making microelectronic connections. A method according to this aspect of the invention includes the step of engaging the tip ends of one or more leads with one or more contacts on a microelectronic component. Here again, the leads have a wettable surface at the tip ends and a non-wettable surface bonding the tip ends. A liquid bonding material is provided at the engaged tip ends and contacts. The liquid bonding material wets the tip ends of the leads and the contacts, but the non-wettable surfaces on the leads confine the liquid bonding material and prevent the liquid bonding material from spreading along the leads from the tip ends. The liquid bonding material is then solidified. For example, where the bonding material includes a eutectic-forming material, the solidification may occur by diffusion between the liquid bonding material and the surrounding solid phases. Where the liquid bonding material includes solder, the solidification may occur upon cooling of the assembly.
The step of providing the liquid bonding material may be performed by providing the bonding material on the tip ends of the leads prior to engaging the tip ends of the leads with the contacts and activating the solid bonding material by heating the assembly while the tip ends are engaged with the contacts. Alternatively or additionally, solid bonding material may be provided on the contacts prior to engagement, and this bonding material also may be activated by heating the assembly while the tip ends of the leads are engaged with the contacts. In yet another alternative, the bonding material may be applied while the tip ends of the leads are engaged with the contacts.
Typically, the leads are provided on a component including a plurality of the leads. Each of the foregoing steps is performed simultaneously for the plurality of leads. For example, plural leads on a single component may be engaged with a semiconductor chip; with a plurality of separate semiconductor chips or with a plurality of semiconductor chips in the form of a unitary wafer so that numerous lead tip ends are engaged with the numerous contacts simultaneously. The methods according to this aspect of the invention provide advantages as discussed above in connection with the components. Thus, here again, confinement of the liquid bonding material improves the strength of the joints between the tip ends of the leads and the contacts, and keeps the bonding material away from the structure of the lead in regions remote from the tips, so that the bonding material does not embrittle the structure.
Yet another aspect of the invention provides methods of making leads. A method in accordance with this aspect of the invention is performed with a lead structure formed in whole or in part from one or more structural metals and including both a tip region and a main region. The method includes the step of treating the main region of the structure to form a surface layer thereon which is non-wettable by a liquid bonding material while leaving at least a portion of the tip region without such nonwettable surface layer and with a surface, wettable by the liquid bonding material. The method may further include the step of applying bonding material adapted to form a liquid phase to the wettable surface in the tip region. The non-wettable surface desirably confines the bonding material to the tip end of the lead when the bonding material is applied.
In one arrangement, the step of treating the main region of the surface includes the step of contacting the main region of the surface with one or more reactants so that the reactants react with one or more of the structural metals in the lead to form the non-wettable surface layer. The method according to this aspect of the invention may further include the step of applying a cover layer on the tip region so as to protect the tip region from the reactant. The cover layer desirably is substantially nonreactive with the reactant under the conditions used in the contacting step. Preferably, the cover layer is an electrically conductive material such as an oxidation-resistant metal as discussed above and the reactant is an oxygen-containing reagent such as an oxygen-bearing plasma, air or other oxygen-containing gas. The cover layer performs a double function: it protects the tip ends of the leads from oxidation or other reactive processing used to form the non-wettable surface on the main region of the lead, and also provides a wettable surface on the tip region of the lead. Methods according to this aspect of the invention can be used to form leads and components as discussed above.
These and other objects, features and advantages of the invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.